Producer Simon Kinberg has admitted he thought 'X-Men: Days of Future Past' would never happen because the film's A-list cast were so difficult to pin down

The producer-and-screenwriter has revealed he initially thought plans for the mutant superhero movie might have to be scrapped because it's A-list cast - which includes Sir Ian McKellan, James McAvoy, Hugh Jackman, Michael Fassbender and Jennifer Lawrence - were so busy and proved tricky to get together at the same time.

Speaking at the long-awaited film's UK premiere in London's Leicester Square last night (12.05.14), Kinberg told BANG Showbiz: "I never thought it would happen because I thought it would be too complicated in terms of cast ... Figuring out the schedules, getting all those people in the same place at the same time was almost as hard as writing it."

The latest instalment in the superhero saga sees Jackman's clawed alter ego travel back in time to prevent the mutant race from becoming extinct, and Kinberg believes focusing the plot so heavily on Wolverine was an obvious choice.

Kinberg reasoned: "Some reasons [for choosing Wolverine] are obvious, he's in some ways the fan favourite of the group in the franchise, the other obvious thing is his age - the character was developed in 1973.

"But for me the part that was the most compelling was the idea that the character who had been saved and mentored by the older Charles Xavier was going back in time was going back in time to save and mentor the younger Charles Xavier, essentially taking the lessons he learnt from the older character and teaching them to his younger self."

Kinberg braved heavy rain to walk the special blue carpet at the event and was joined by several stars of the film such as Fassbender, McKellen and McAvoy who was accompanied by his actress wife Anne-Marie Duff.

When the mutants of the future face extinction, they send Wolverine (Jackman) back in time to change destiny. The finale is too effects-heavy and non-fans may question whether it all makes sense, but the principal players are all on top form and the slick direction and smart writing are at times enormously entertaining.